在SQL中模拟环（代数）

Question

I'm creating small profile viewer and I want to simulate ring in SQL. How it works? For simple: I have 5 users on my table. When I get one from middle to my experiment (like user number 2,3,4) next and previous user is CURRENT_USER_ID +- 1. But when I choose user number 1 or 5 it's not that simply like before. So if I have some tool like algebra ring it's easy. Can I simulate it?

Oh, I'm using MySQL.

Answer 1

Like this: SQLFIDDLE

set @curr_user = 1;
set @maxid = (select max(u1.id) maxid from users u1);
set @minid = (select min(u2.id) minid from users u2);
set @next_user = (if(@curr_user = @maxid
                           ,@minid
                           ,@curr_user +1));
set @prev_user = (if(@curr_user = @minid
                           ,@maxid
                           ,@curr_user - 1));

SELECT t1.name as prevuser, t2.name as curruser, t3.name as nextuser
FROM users t1
join users t2
on 1=1
JOIN users t3
on 1=1
WHERE t1.id = @prev_user
AND t2.id = @curr_user
and t3.id = @next_user

EDIT: I made a slight adjustment so it would work even if there are missing id's, let's say you have user 1,3,4,5,7 this would get user 4,5,7 if current is 5 SQLFIDDLE

set @curr_user = 3;
set @maxid = (select max(u1.id) maxid from users u1);
set @minid = (select min(u2.id) minid from users u2);
set @nextid = (select min(id) from users where id > @curr_user );
set @previd = (select max(id) from users where id < @curr_user );
set @next_user = (if(@curr_user = @maxid
                           ,@minid
                           ,@nextid));
set @prev_user = (if(@curr_user = @minid
                           ,@maxid
                           ,@previd));

SELECT t1.name as prevuser, t2.name as curruser, t3.name as nextuser
FROM users t1
join users t2
on 1=1
JOIN users t3
on 1=1
WHERE t1.id = @prev_user
AND t2.id = @curr_user
and t3.id = @next_user